Hole Transport Materials
One class of hole transport materials comprises aromatic tertiary amines including at least two aromatic tertiary amine moieties (e.g. those based on biphenyl diamine or of a “starburst” configuration), of which the following are representative and of which at this time α-NPB (formula in the specific description below) is believed to be the most widely accepted and used in commercial production.
WO 2011/021803 (Duksan) discloses compounds having a thianthrene structure and their use in OLEDs. In examples, the five compounds below were synthesized.

The above compounds were tested as host materials forming part of a doped electroluminescent layer in an OLED. A layer of 10 nm copper phthalocyanine on an ITO electrode had deposited thereon 30 nm of α-NPB (also known as α-NPD) as hole transporter, a layer of one of the above thianthrene compounds or of CBP to serve as host material for Ir(ppy)3 as dopant, 10 nm of aluminium biphenoxy bis(2-methyl quinolate) as hole blocker, 40 nm of aluminium quinolate as electron transporter, 0.2 nm of lithium fluoride as electron injector and aluminium as cathode. Green electroluminescence with substantially the same colour coordinates was obtained when the test thianthrene compounds were used as host as when CBP was used as host, and turn-on voltage and luminous efficiency (cd/A) ranged from slightly worse than with CBP to somewhat better. However, the fused carbazole ring structures of the Duksan compounds exhibit relatively low hole mobility so that these compounds would be expected to exhibit poorbried performance if used as hole transporters. It should be mentioned that CBP, which also has carbazole rings linked directly to an extended aromatic system, has relatively low hole mobility and is also better as a host material in an electroluminescent layer than as a hole transport layer material. It is unsurprising, therefore, that Duksan employs a conventional hole transporter and does not employ any of the thianthrene compounds for that purpose.
Functionalised thianthrenes alleged to have hole transport properties and alleged to be blue emitters are disclosed by Świst et al., ARKIVOC 2012 (iii), 193-209 (2012). Suzuki coupling of thianthren-2-yl-2-boronic acid with a variety of brominated aromatic amino compounds having C4H9, C12H25 or C16H33 substituents gave oils or, in one instance, a solid of low melting point. Stille coupling of 2,8-dibromothianthrene with (Bu)3Sn— derivatives of thiophene, oxazole, furan and pyridine gave 2,8-bis(2-oxazolyl) thianthrene which was an oil, 2,8-bis(2-thiophenyl) thianthrene m.p, 176-179° C., 2,8-bis(2-furanyl)thianthrene m.p. 204-206° C. and 2,8-bis(2-pyridyl)thianthrene (m.p. 113-114° C., all of which are undesirably low for device applications. Although the compounds were investigated by cyclic voltammetry, DPV spectroscopy, UV-Visible spectroscopy and fluorescence—were alleged to have band gaps in a range appropriate for semiconductors, no hole mobility measurements were made, and the compounds were not tested in OLEDs or other practical devices. They are alleged to be castable into uniform films, but this would not be a property shared by those compounds which are oils. There is no disclosure or suggestion that the materials reported, or any of them, should be used as hole transport layers in OLEDs as opposed to alternative devices such as organic photovoltaic devices, or that any of them give better properties in a hole transport layer of an OLED than established materials e.g. α-NPB and no reason to suppose that this is the case.
U.S. Pat. No. 8,012,606 (Takahiro et al, Nippon Steel) discloses heterocyclic compounds represented by the general formulae
wherein: R represents a hydrogen atom, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group, Ar1, Ar2, Ar3, and Ar4 denote independently a substituted or unsubstituted aryl group or Ar1, Ar2 together with the nitrogen atom bonded thereto or Ar3, Ar4 together with the nitrogen atom bonded thereto may form a nitrogen-containing hetero ring (e.g. N-carbazolyl, N-phenoxazinyl, N-phenothiazinyl), and m and n are independently 1 or 2. Examples disclose compounds as described above serving as components of the light-emissive layer of an OLED and in one instance as a hole transport layer. The compounds are alleged when applied to an organic EL device to enables the device to be driven at low voltage. When used as a host material, electrons and holes are alleged to move in a well-balanced way to form a wide range of emission of light and attain high luminous efficiency. Furthermore, the heterocyclic compounds have a high triplet energy which is important in an electroluminescent device utilizing phosphorescence. Hence, when used as a host material or an electron-transporting material for a phosphorescent device, the energy of the triplet excited state of a phosphorescent dopant can be confined efficiently and phosphorescence can be obtained at high efficiency. In addition to these good electrical properties, the said heterocyclic compounds are alleged to be stable when formed into thin film. An organic EL device comprising the heterocyclic compound of this invention in its organic layer efficiently emits light of high brightness at low voltage and shows excellent durability and it is applicable to flat panel displays Representative compounds include:    2,7-bis(phenylamino)dibenzodioxin,    2,7-bis(9-carbazolyl)dibenzodioxin,    2,7-bis(N-3-biphenylyl-N-phenylamino)dibenzodioxin,    2,7-bis(N-1-naphthyl-N-phenylamino)dibenzodioxin and    2,7-bis(9-carbazolyl) thianthreneLight Emission
Materials that emit light when an electric current is passed through them are well known and used in a wide range of display applications. Devices which are based on inorganic semiconductor systems are widely used. However these suffer from the disadvantages of high energy consumption, high cost of manufacture, low quantum efficiency and the inability to make flat panel displays. Organic polymers have been proposed as useful in electroluminescent devices, but it is not possible to obtain pure colours; they are expensive to make and have a relatively low efficiency. Another electroluminescent compound which has been proposed is aluminium quinolate, but it requires dopants to be used to obtain a range of colours and has a relatively low efficiency.
Patent application WO 98/58037 describes a range of transition metal and lanthanide complexes which can be used in electroluminescent devices which have improved properties and give better results. Patent Applications WO 98/58307, WO 00/26323, WO 00/32719, WO 00/32717, WO 00/32718 and WO 00/44851 describe electroluminescent complexes, structures and devices using rare earth chelates. U.S. Pat. No. 5,128,587 discloses an electroluminescent device which consists of an organometallic complex of rare earth elements of the lanthanide series sandwiched between a transparent electrode of high work function and a second electrode of low work function, with a hole conducting layer interposed between the electroluminescent layer and the transparent high work function electrode, and an electron conducting layer interposed between the electroluminescent layer and the electron injecting low work function cathode. The hole conducting layer and the electron conducting layer are required to improve the working and the efficiency of the device. The hole transporting layer serves to transport holes and to block the electrons, thus preventing electrons from moving into the electrode without recombining with holes. The recombination of carriers therefore mainly takes place in the emissive layer.
JP 2005-314239 Mitsui Chemicals discloses compounds said to be suitable for incorporation into electroluminescent layers of OLED devices and having thianthrene bonded to anthracene. However, there is no disclosure or suggestion of the suitability of incorporating such compounds into an electron transport layer of such a device, and in exemplified cells the electron transport layer used was of well-known materials such as aluminium quinolate.
In order to enhance the performance of electroluminescent organometallic complexes the electroluminescent organometallic complex can be mixed with a host material and we have now devised an improved electroluminescent material using a metal quinolate as the host material.
Electron Transport Materials
Kulkarni et al., Chem. Mater. 2004, 16, 4556-4573 (the contents of which are incorporated herein by reference) have reviewed the literature concerning electron transport materials (ETMs) used to enhance the performance of organic light-emitting diodes (OLEDs). In addition to a large number of organic materials, they discuss metal chelates including aluminium quinolate, which they explain remains the most widely studied metal chelate owing to its superior properties such as high EA (˜−3.0 eV; measured by the present applicants as −2.9 eV) and IP (˜−5.95 eV; measured by the present applicants as about −5.7 eV), good thermal stability (Tg ˜172° C.) and ready deposition of pinhole-free thin films by vacuum evaporation. Aluminium quinolate remains a preferred material both for use as a host to be doped with various fluorescent materials to provide an electroluminescent layer and for use as an electron transport layer. For a hole transporter or electron transporter to work effectively in a phosphorescent device, the triplet levels of the respective materials should be higher than the triplet level of the phosphorescent emitter.